1. Field
The present disclosure relates to an integrator, a touch sensing system using the same, and a display device coupled with the touch sensing system.
2. Related Art
User interfaces (UI) enable humans (users) to interact with various types of electric or electronic devices so that they can easily control the devices as they want. Typical examples of the user interfaces include keypads, keyboards, mice, on-screen displays (OSD), and remote controllers with an infrared communication capability or radio frequency (RF) communication capability. The user interface technology is continuing to make progress toward higher user sensitivity and ease of operation. Recently, user interfaces have been evolving into touch UI, voice recognition UI, 3D UI, etc.
Capacitive touchscreens can be implemented as capacitance sensors. The capacitance sensors may be classified into self-capacitance sensors and mutual capacitance sensors.
As shown in FIG. 1, a mutual capacitance sensor includes mutual capacitance CM formed between two electrodes Tx and Rx. A sensing part 12 applies a driving signal (or stimulus signal) to Tx lines Tx1 to Tx5, and senses touch input based on a change in the amount of charge in the mutual capacitance before and after a touch on Rx lines Rx1 to Rx6. The change in the amount of charge in the mutual capacitance CM refers to the difference in the amount of charge before and after touch input. The mutual capacitance CM decreases when a conductive object is brought closer to it. The sensing part 12 converts the change in the amount of charge to digital data (hereinafter, referred to as ‘touch raw data’) by an analog-to-digital converter (hereinafter, referred to ‘ADC’) and outputs it.
As shown in FIG. 2, a self-capacitance sensor includes self-capacitance Cs formed in each sensor electrode. A sensing part 14 supplies charge to each sensor electrode and senses touch input based on a change in the amount of charge in the self-capacitance Cs. The self-capacitance Cs increases when a conductive object is brought closer to it. The sensing part 14 converts the change in the amount of charge to touch raw data by an ADC and outputs it.
The sensing parts 12 and 14 sample a change in the amount of charge received from touch sensors as a voltage by using a charge amplifier and an integrator. The output voltage of the integrator is input into the analog-to-digital converter (hereinafter, referred to as ‘ADC’) and converted to digital data (hereinafter, referred to as ‘touch raw data’).
The charge amplifier outputs a change in the amount of charge as a voltage in the rising period of a driving signal applied to the touch sensors. The integrator amplifies the difference between the output voltage of the charge amplifier and a reference voltage of the integrator at the ratio of α=Cs/CFBI. Herein, Cs is the capacitance of a sampling capacitor, and CFBI is the capacitance of a feedback capacitor of the integrator. Then, the integrator accumulates the sampled voltage in the capacitor CFBI and integrates it.
The conventional integrator has the following problems.
First, the number of integrations is limited because the voltage of the charge amplifier, which is integrated by the integrator, is higher than necessary. In an example shown in FIG. 3, a touch can be detected based only on the difference |V1−V1_t| between a non-touch voltage V1 and a touch voltage V1_t; however, the integrator integrates a voltage higher than necessary since the output voltage Vout of the charge amplifier is higher than the voltage difference |V1−V1_t|. The output voltage Vout of the charge amplifier is V1 when there is no touch, and V1_t when there is touch input. In FIG. 3, Vref is the reference voltage of the charge amplifier.
Second, the integrator has a larger circuit area when it is made capable of double sampling. The integrator may be implemented as a single-ended amplifier or a differential amplifier. The integrator may be implemented as a differential type Then, the integrator accumulates the sampled voltage in a capacitor CFBI and integrates it. integrator capable of double sampling. This differential type integrator requires more than twice as many integrators and switches as the single-ended integrator.
Third, the output voltage range of the integrator is narrow. The output voltage of the integrator ranges from 0 to VDD/2, where VDD is the supply voltage of the integrator, because the reference voltage of the integrator is VDD/2.